1. Field of the Invention
The present invention relates to a data transmitting apparatus and a data transmitting method for transmitting a plurality of real time streams and a non-real time stream over one transmission path.
2. Description of the Related Art
When a plurality of real time streams are transmitted over one transmission path, the transmission times of packets of the real time streams are kept nearly constant. In other words, a predetermined time interval is set for each stream. Time intervals of individual streams are measured. After the time interval of each stream has elapsed, packets thereof are transmitted. When packets of each stream are transmitted at the predetermined time intervals, the time intervals of packets of each stream become nearly constant. Thus, a predetermined transmission rate can be kept constant.
While packets of one stream are being transmitted over one transmission path, packets of another stream cannot be transmitted thereonto. Thus, when packets of a plurality of streams have to be transmitted over one transmission path, it is necessary to mediate them. Only packets of a stream that has been mediated are permitted to be transmitted. Packets of the other streams are placed on standby.
Thus, as described above, to keep the transmission rate of each stream constant, the transmission times of packets of each real time stream are kept nearly constant. However, actually, the transmission times of packets of each stream delay (namely, jitters take place). Such jitters cause a screen to distort when image data in accordance with the Moving Picture Coding Experts Group (MPEG) or the MPEG2 is transmitted.
When packets of a plurality of streams are transmitted at a time, the round robin method is used to mediate them.
FIG. 1 shows a structure of a conventional stream transmitting apparatus that transmits a plurality of real time streams and a non-real time stream over one transmission path.
In FIG. 1, buffer memories 101, 102, 103, and 104 are First-In First-Out (FIFO) memories. Packets of the streams are successively stored in the buffer memories 101, 102, 103, and 104. When packets of the streams have been stored in the buffer memories 101, 102, 103, and 104, they send transmission requests to a round robin portion 105. When the transmission times of packets transmitted from the buffer memories 101, 102, 103, and 104 overlap, the round robin portion 105 mediates the transmission requests.
FIG. 2 shows a mediating method performed by the round robin portion 105. In FIG. 2, the round robin process is reset (at step S101). A packet that has been stored first in a buffer memory is transmitted.
Thereafter, it is determined whether or not there is a packet in the buffer memory (at step S103). When there is another packet in the buffer memory, the buffer memory that has transmitted a packet is prohibited from transmitting another packet. A buffer memory that has not transmitted a packet is given a transmission right (at step S104). Thereafter, it is determined whether or not all the buffer memories have transmitted packets (at step S105). Until all the buffer memories have transmitted packets, steps S103 to S105 are repeated.
When a real time stream and a non-real time stream such as MPEG streams are transmitted over one transmission path, as a related art reference that prevents packets from colliding, Japanese Patent Laid Open Publication No. 2001-86499 titled “Image Transmitting Apparatus and Image Transmitted Method (translated title)” filed by Victor Company of Japan, Ltd. is known.